Text Box: STANDARD REPAIR PROCEDURE USED BY THIS AGENCY This facility performs all repairs under guidelines established by the FAA (FAR 43 appendix F is the best example), and factory specifications by the manufacturer of the equipment.  Tech data is kept current. Test equipment is on a calibration schedule. The Repair Station environment requires several ongoing inspections.  The preliminary (or incoming) inspection is generally a visual and paperwork inspection where proper tech data is established and a workorder is generated. When the unit reaches the bench it is subjected to testing and troubleshooting procedures outlined by the unit's manufacturer, which vary, but are generally as follows: The unit is connected to a bench harness connected to a DC power supply and an altitude (mode C) simulator.  A second connection is made to the antenna input/output, and this is connected to the transponder test set.  Initial testing establishes the receiver sensitivity, transmitter power, frequency stability, etc.  Also measured is the information the transponder transmits. Based upon the information you have given us when you order service, and our own evaluation, we are ready to troubleshoot. Transponders contain several discrete circuits in addition to the receiver & transmitter.  We employ both dynamic (power on) and passive (power off) troubleshooting techniques to isolate the problem component.  Other methods such as intermittent and thermal intermittent testing may be used. Usually we find just one bad component, the "weakest link in the chain".  Less often we encounter a multiple failure where one malfunctioning part will infect another (that's why we like to hear if the unit has been recently serviced). Often the most time consuming portion is replacing a defective component.  We accomplish this by prepping the board and using specialized de-soldering equipment.  Our goal is a quick turn-around so we keep a good parts inventory. We always perform a thorough factory specified alignment before final inspection.  Certain transponders may go through several alignments as part of the troubleshooting procedure. Sometimes an alignment is the troubleshooting procedure. Final inspection is the test that matches your freshly repaired transponder to factory specifications.  The unit is 'returned to service' (paperwork completed), boxed and ready to ship.